Eleven families of phosphodiesterases (PDEs) have been identified but only PDEs in Family I, the Ca2+-calmodulin-dependent phosphodiesterases (CaM-PDEs), which are activated by the Ca2+-calmodulin and have been shown to mediate the calcium and cyclic nucleotide (e.g. cAMP and cGMP) signaling pathways. The three known CaM-PDE genes, PDE1A, PDE1B, and PDE1C, are all expressed in central nervous system tissue. PDE1A is expressed throughout the brain with higher levels of expression in the CA1 to CA3 layers of the hippocampus and cerebellum and at a low level in the striatum. PDE1A is also expressed in the lung and heart. PDE1B is predominately expressed in the striatum, dentate gyrus, olfactory tract and cerebellum, and its expression correlates with brain regions having high levels of opaminergic innervation. Although PDE1B is primarily expressed in the central nervous system, it may be detected in the heart. PDE1C is expressed in olfactory epithelium, cerebellar granule cells, striatum, heart, and vascular smooth muscle.
Neurogenesis is a vital process in the brains of animals and humans, whereby new nerve cells are continuously generated throughout the life span of the organism. The newly born cells are able to differentiate into functional cells of the central nervous system and integrate into existing neural circuits in the brain. Neurogenesis is known to persist throughout adulthood in two regions of the mammalian brain: the subventricular zone (SVZ) of the lateral ventricles and the dentate gyrus of the hippocampus. In these regions, multipotent neural progenitor cells (NPCs) continue to divide and give rise to new functional neurons and glial cells (for review Gage 2000). It has been shown that a variety of factors can stimulate adult hippocampal neurogenesis, e.g., adrenalectomy, voluntary exercise, enriched environment, hippocampus dependent learning and antidepressants (Yehuda 1989, van Praag 1999, Brown J 2003, Gould 1999, Malberg 2000, Santarelli 2003). Other factors, such as adrenal hormones, stress, age and drugs of abuse negatively influence neurogenesis (Cameron 1994, McEwen 1999, Kuhn 1996, Eisch 2004).
While the importance of neurogenesis cannot be overstated, the failure of axons to regenerate after spinal cord injury still remains one of the greatest challenges facing both medicine and neuroscience. An important development, however, has been the identification of inhibitory proteins in CNS myelin. One problem that causes the failure of CNS neuron regeneration is inhibition of neurite outgrowth by certain bioactive molecules. Myelin contributes to a number of proteins that have shown to inhibit neurite process outgrowth. NogoA is the first protein identified on the surface of the oligodendrocytes and some axons. Other proteins that can contribute to inhibition include myelin-associated glycoprotein (MAG), oligodendrocyte-myelin glycoprotein (OMgp) and the proteoglycan versican.
It is believed that the central nervous system (CNS) environment could limit axonal regeneration after injury. Indeed, CNS myelin has been identified as a major factor contributing to regenerative failure. There are those in the field that believe, and have provided evidence, that CNS myelin contains proteins that inhibit axonal growth.
Various strategies have been proposed for overcoming myelin inhibition. One strategy that has been effective has been to elevate the levels of intracellular cAMP. Some manners in which this may be done include: a peripheral conditioning lesion, administration of cAMP analogues, priming with neurotrophins or treatment with the phosphodiesterase inhibitor rolipram (PDE4 inhibitor). The effects of cAMP may be transcription dependent, and cAMP-mediated activation of CREB may lead to upregulation and expression of genes such as arginase I and interleukin-6. The products of these genes are believed to promote axonal regeneration, which raises the possibility that other cAMP-regulated genes could yield additional agents that would be beneficial in the treatment of spinal cord injury. However, with regard to increasing the expression of IL-6, one significant disadvantage to this mechanism of action may be that IL-6 is a potentially harmful pro-inflammatory cytokine, meaning, it is possible that high levels of IL-6 could actually exacerbate the inflammation that occurs after spinal cord injury which could then lead to increase in cell death. Indeed, a factor supporting this concern is that IL-6 transgenic mice have been observed to have extensive astrogliosis, neurodegeneration, and breakdown of the blood brain barrier.